Electron beam processing or electron processing technology (EPT) of chemical wood pulp has been known and studied for many years, particularly in eastern European countries. Such studies have included laboratory and plant scale tests of viscose and fibre quality.
Dissolving pulp is electron treated off-line before going to a viscose process. High energy electrons interact with cellulose resulting in two main effects, namely cellulose activation and degree of polymerisation (DP) reduction. The aforementioned effects can give rise to significant benefits in a viscose process such as savings in chemical consumption (of compounds such as carbon disulfide, sodium hydroxide and sulphuric acid), and a reduction in environmental pollution in the pulping process and potentially also environmental benefits in a customers process.
As far as the applicant is aware, the earliest patent in this field was known as the Wolfen patent, with inventors Fisher K, Goldberg W and Wilkie M and was granted to the company Dresden/Fisher (DE 2941624/DD WP208708). The aforementioned patent described the benefits of electron beam treatment for the viscose process in terms of chemical savings due to the activating effect of the treatment.
Since the aforementioned patent, a number of patent applications and granted patents have been published. Consequently the field of EPT for pulp and viscose pulp activation has been well researched and explored, and hence is well known.
The higher the electron beam energy, obviously the more penetrating it is. The lowest beam energy that is of practical use is about 250 keV but there is probably no upper limit. The present practical upper limit is probably 50 MeV as no larger machines exist, but more probably is 12 MeV as above this level radioisotopes can form. A further possible limiting factor for upper energy levels is that, the higher the energy, the thicker the shielding that is required. For example, a 300 keV machine can be used effectively unshielded (—it is self shielded with a lead coating on the surface of the machine).
This type of technology is typically used, for example for curing surface coatings but anything of higher energy typically needs proper (external) shielding which is typically made of concrete for machines above about 500 keV.
Another limiting factor in EPT of chemical woodpulp is that such treatment to-date has been carried out on an off-line basis which increases processing cost by way of additional pulp handling, and by way of additional processing or production time.
An observation when using high energy electrons, or even gamma rays, is that their effect is to break down the molecular weight of the cellulose and polymer in a non-linear manner with respect to the applied dosage. An initial dose gives rise to a large reduction in DP which then levels off as dosage is increased. Apparently it has been assumed by workers in this field—and there does not appear to be any literature to suggest otherwise—that the starting DP needs to be known accurately in order to arrive at an accurate final DP.
The present method of off-line pulp processing not only requires that the starting DP be known but is also prone to variability of product when pulp stacks are used due to variable heights and to edge to middle of stack effects. It also produces a product which is inherently more variable and of lower ‘quality’ than a single sheet treatment. For single sheet processing off-line the level of handling required makes the costs excessive and uneconomic.